Modem communication systems, such as cellular telephones, generally communicate using radio frequency (RF) signals. Voice/data input into a cellular telephone is a low frequency or baseband signal. A transmitter converts the baseband signal to a radio frequency signal for transmission by the cellular telephone. The baseband signal is passed through a low frequency stage, converted to a radio frequency, and passed through a high frequency stage.
The baseband signal can be converted using any of a variety of modulation techniques. IQ modulation, also known as Quadrature Amplitude Modulation (QAM), is a method for sending two separate (and uniquely different) channels of information where the carrier signal is shifted to create two carriers—sine and cosine versions. Conventionally, two modulation inputs are applied to two separate modulators, each of which are supplied with sine or cosine carriers. The outputs of both modulators are algebraically summed; the result of which is a single signal to be transmitted, containing the I and Q information. On receipt, the composite signal (I and Q) is processed and extracted by the receiver using a carrier replica (sine and cosine).
Switched-mode mixers or modulators have been shown to present improved noise, linearity, and current-drain performance. Heretofore, switched-mode modulators have only been able to operate on a single signal, such as amplitude modulation (AM). If the output of two such modulators were resistively summed to produce an in-phase and quadrature (“IQ”) modulator, a 50% reduction in efficiency results over the underlying AM modulator since the I and Q signals oppose each other on average 50% of the time.
U.S. Pat. No. 7,194,242 to Tanaka entitled “Direct-Conversion Transmitter Circuit and Transceiver System” describes two sets of Gilbert Cell mixers with current summation to produce an IQ-modulated signal. Gilbert Cell mixers are popular double-balanced mixers used in RFIC (radio frequency integrated circuit) designs. The Gilbert Cells require large amounts of current in order to maintain linearity and low noise. There is also a significant wasting of current in such designs. U.S. Pat. No. 6,587,010 to Wagh, et al. entitled “Modulated radio frequency signal generation method and modulated signal source” describes a system where input I and Q signals are encoded as 1-bit signals using pulse width modulation, pulse density modulation, etc. However, this system does not provide for analog inputs.